Is 18 a prime number?
18 is not a prime number. Here is a way to check it.
The answer
No. 18 is not a prime number: 2 × 9 = 18.
How to see that 18 is not prime
18 is even, and every even number can be divided by 2, so 2 is a factor as well as 1 and 18.
18 has 6 factors (1, 2, 3, 6, 9 and 18), which make 3 factor pairs.
Split 18 into prime factors
Split off the smallest prime each time, and keep going until every part is prime:
- 18 = 2 × 9
- 18 = 2 × 3 × 3
With repeated primes written as powers, that is 2 × 3². A small raised number tells you how many times to use that factor: for example, 2³ means 2 × 2 × 2.
18 in the times tables
18 turns up in 4 different times tables, the 2, 3, 6 and 9: 2 × 9, 3 × 6, 6 × 3 and 9 × 2.
Quick checks that catch 18
- The last digit, 8, is even, so 2 goes in.
- The digits add up to 9, which is in the 3 times table, so 3 goes in.
- It is even and passes the check for 3 as well, so 6 goes in.
- The digit total, 9, is a multiple of 9, so 9 goes in.
18 at a glance
Optional extra number facts:
- In words
- eighteen
- In Roman numerals
- XVIII
- Odd or even
- Even: it ends in 8
- Its factors add up to
- 39, or 21 without 18 itself. 18 is abundant: Abundant means this sum is greater than the number.
The numbers either side of 18
- 17 is prime: 2 and 3 do not go into it, and 5 × 5 is already past 17. Check 17 in full
- 19 is prime: 2 and 3 do not go into it, and 5 × 5 is already past 19. Check 19 in full
Turn the digits round
18 backwards is 81. 81 is not prime. Its digits add up to 8 + 1 = 9, which is a multiple of 3. That means 81 is a multiple of 3 too, so 3 is a factor. Check 81 in full
Related pages
More numbers
Questions parents ask
Which prime numbers are either side of 18?
17 below it and 19 above it, a gap of 2.
What is 18 squared?
18 × 18 = 324.
What is the smallest number with as many factors as 18?
12. It has 6 factors, the same as 18.
Continue your number practice
Explore the diagnostic for another set of practice questions.